1. Field of the Invention
This invention relates to sanitary tissue products.
2. Description of the Prior Art
Sanitary tissue products, which are also called bathroom tissue and toilet tissue products, generally fall into one of four categories based on their composition and structure: One ply of conventional paper web; two-ply laminate of conventional paper webs; one ply of high bulk process, low-density paper web such as produced by the process described in U.S. Pat. No. 3,301,746 issued to Sanford and Sisson on Jan. 31, 1967; and two-ply laminate of high bulk process paper webs. These products are usually produced and marketed in roll form with individual sheets of the tissue defined by perforations extending across the width of the tissue product which is wound lengthwise on a hollow core.
Sanitary tissue products are generally evaluated by consumers in terms of softness, absorbency, flexibility, bulk, and, to a lesser degree, strength. Softness is that quality of the tissue manifested by a cushioned, resilient, non-harsh, pleasant sensation to the fingers or other portions of the human anatomy. Absorbency is, as the term applies, the ability of the tissue to absorb moisture. Flexibility is the ease with which the tissue bends during use. Bulk is a measure of the thickness of the sheet and is manifested in several ways such as by the tactile sensation produced by the tissue and by the diameter of a roll of a given number of standard size sheets. Bulk is, of course, inversely related to density. Strength is the ability of the sanitary tissue to resist tearing during use. Strength is also manifested by the ability of the sanitary tissue to be separated into its individual sheets by disjointing the web at the perforations without tearing the remainder of the sheet. While the five attributes hereinbefore mentioned have been treated as separate entities, they are, in fact, interrelated. For example, bulk contributes to the softness and softness impression of the sanitary tissue as does flexibility. Bulk also enhances the absorbency impression of the tissue as well as being objectively related to absorbency for physical reasons.
FIG. 1 is a representation of the cross-section of a sheet of creped paper web 11 used in making sanitary tissues. In this drawing, the thickness of the material and the degree of creping are both highly exaggerated. It is possible to define two thicknesses for this paper web. The first, represented by T in FIG. 1, is the true, inherent thickness of the web. The maximum distance between the tops of the peaks and the bottoms of the valleys of the creped web, as represented by t in FIG. 1, is a more practical measure of thickness and is denominated caliper. Caliper is used throughout this specification and is measured under load as hereinafter described.
Sanitary tissue products made from one ply of conventional paper web are generally characterized as being harsh and stiff. They are especially noted for their lack of absorbency. All three of these less than desirable attributes result from the necessity of making a paper web which is strong enough to withstand ordinary use. In general, there are two ways to increase the strength of a paper web: using synthetic resins and increasing the basis weight (weight per standard area) of the web. Since sanitary tissue must be readily dispersable in water, the use of synthetic resins is limited to dry strength resins which significantly increase the harshness of a paper web. In sanitary tissues, the strength of the web is generally increased by increasing the basis weight of the web; increasing the basis weight of a paper web increases its stiffness and harshness. In addition, the bulk of a one ply conventional paper web product is, and is perceived as being, low because of the inherent inability of conventional paper machines to make a paper web of low density. As indicated above, the lack of bulk is related to the poor absorbency of this tissue product.
Sanitary tissue products constructed of one ply of high bulk process paper web are generally considered to be an improvement over one ply conventional paper web sanitary tissues in terms of softness and absorbency. This improvement generally results because of the ability of the high bulk process (hereinafter described) to make a paper web of significantly lower density than that which may be made on conventional paper machines. Lower density inherently leads to greater absorbency and greater softness. Lower density also, of course, inherently means greater bulk which not only adds to the softness impression of the product but also results in a much thicker roll of product. Since decreased density inherently results in decreased strength of the web, the best combination of softness and absorbency is impossible to obtain with a single ply high bulk process tissue because of the necessity of maintaining adequate product strength.
Sanitary tissue products made from two conventional paper webs laminated to form a two-ply tissue product are generally softer, more absorbent, and have greater bulk impression overall than do sanitary tissue products made from a single ply of either conventional or high bulk process webs. The desirable qualities of the two-ply conventional web product results in the ability to use webs of lower basis weight than can be used in one ply products. As noted above, a web of lower basis weight generally results in a softer, more flexible web than does a higher basis weight. The strength of the web is reduced as basis weight is reduced, but by laminating two webs together it is increased to an adequate level. As is well known to those skilled in the art, laminating two webs to make a product generally increases the absorbency of the product. It must be noted that laminating two webs of a given caliper does not result in a product having a caliper twice that of the individual web. As is well known, and as shall be more fully discussed hereinafter, there is a caliper loss on lamination.
Sanitary tissues made from two plies of high bulk process paper web are generally considered to be an improvement over products made from two plies of conventional paper webs. The same considerations, discussed above, that combine to result in increased softness and absorbency when two conventional webs are laminated to form a single sanitary tissue product apply when two high bulk process webs are so laminated. The use of two-ply sanitary tissues made from high bulk process paper webs is not more common because of problems of supply. Paper machines designed and constructed to make high bulk process webs are very expensive, as are all paper machines, and are not yet common throughout the paper industry. Consequently, the supply of high bulk process paper is not yet adequate to supply the demand for both one ply and two ply products. Economic considerations have dictated that the major part of the supply of high bulk process paper be used in one-ply sanitary tissues even though these products are somewhat less desirable in certain attributes than are two-ply tissues made from the high bulk process web. At the same time, there is excess capacity available on conventional paper machines.